The goal of the proposed research is to elucidate the mechanism whereby vasopressin promotes transcellular water movement in responsive epithelial tissues. The specific objective is to determine: 1) the role of microtubules and microfilaments in this action of vasopressin, and 2) to examine the possiblity that cytosolic calcium ions play a role in the action of the hormone on transepithelial water and sodium transport. Functional studies on the movement of water and sodium in response to vasopressin will be carried out in isolated toad bladders; the effects of procedures which may influence (1) microtubule structure and/or function; (2) the cytosolic calcium ion concentration will be examined. Identification of actin in toad bladder epithelial cells will be attempted. Light and electron microscopic studies on the state of assembly and distribution of microtubules, and of actin filaments, in the bladder epithelial cells will be carried out, using immunocytochemical techniques.